In the case of the Painted Lady and the Large White, the lack of evidence for a return migration puzzled Williams:
I have already (Williams et al. I942, p. 250) pointed out the difficulty of accounting for the persistence of a habit of movement in one direction only on any theory of evolution. If we accept that directional movement in butterflies is simply an overflow from an over-populated area, and that none of the emigrants ever return; then the species must be perpetuated by the offspring of those individuals which do not emigrate. Thus a habit must persist for countless generations, in spite of the fact that all individuals which develop it die without contributing to the continuity of the species.He was optimistic though, and correctly predicted that return flights would be found in all species once enough research effort was put into it, making butterfly migration as important a phenomenon as that in birds. Indeed, recent research - some of it carried out in Rothamsted - including meteorological analysis of prevailing winds when influxes of Painted Ladies occur, and entomological radar research has shown that the Painted Lady taked advantage of tailwinds as a migratory aid and that the reason that these flights were not detected is that they fly at high altitude.
Migrating butterflies are tracking suitable breeding grounds, areas where larval food plants are abundant. Red Admiral caterpillars feed on fresh, rapidly growing nettles. At the end of spring, nettles start to wither in the Mediterranean winter grounds, so the spring Red Admiral adult generation flies to Northern Europe - and also up mountains - where nettles are luscious. They mate and lay eggs in the north and the resulting autumn generation of adults migrates due south in the autumn.
Unlike birds, butterflies breed both in the winter and summer grounds, and also unlike birds, each generation makes a one way trip, leaving the return journey to their offspring.
(Photo above, a fresh, autumn generation Red Admiral feeding on Cherry Laurel, 19 Aug 2011)
Williams, C. (1951). Seasonal Changes in Flight Direction of Migrant Butterflies in the British Isles The Journal of Animal Ecology, 20 (2) DOI: 10.2307/1537
Kauri Mikkola (2003). The red admiral butterfly (Vanessa atalanta) is a true seasonal migrant: an evolutionary puzzle resolved? European Journal of Entomology, 100, 625-626
Stefanescu, C., Alarcon, M., & Àvila, A. (2007). Migration of the painted lady butterfly, Vanessa cardui, to north-eastern Spain is aided by African wind currents Journal of Animal Ecology, 76 (5), 888-898 DOI: 10.1111/j.1365-2656.2007.01262.x
Chapman, J., Nesbit, R., Burgin, L., Reynolds, D., Smith, A., Middleton, D., & Hill, J. (2010). Flight Orientation Behaviors Promote Optimal Migration Trajectories in High-Flying Insects Science, 327 (5966), 682-685 DOI: 10.1126/science.1182990